The present invention relates to a sheet handling apparatus equipped with sheet separating means for separating sheets stacked on a sheet stacker into a single sheet and feeding the single sheet toward a downstream side. More particularly, the present invention relates to a sheet handling apparatus and an image reading apparatus that detect whether two or more sheets are overlapped and transported (double feed).
An image reading apparatus is mounted on a copier or facsimile machine for separating sheets (such as cut sheets or plastic sheets) stacked on a sheet stacker. In a case of a copier, an image reading apparatus separates sheets from a cassette into a single sheet and feeds the single sheet to a printing unit, so that characters or images are printed on the sheet.
In such a sheet handling apparatus for reading images on originals or printing, it is technically important to securely separate sheets stacked on a sheet stacker such as an original tray or a paper cassette and feed a single sheet one at a time. Accordingly, an automated document feeder (hereinafter referred to as ‘ADF’) is provided with a sheet separating mechanism for separating sheets stacked on a sheet stacker (feeder tray) into a single sheet, so that the automated document feeder feeds the single sheet one at a time. Japanese Patent Publication (Kokai) No. 2001-354339 has disclosed an example of such a sheet separating mechanism.
In the sheet separating mechanism disclosed in Japanese Patent Publication (Kokai) No. 2001-354339, a draw roller capable of rising and lowering touches the uppermost sheet stacked on the sheet stacker to draw the original (sheet). A feed roller is provided for feeding only the uppermost original (sheet), and a separating pad is provided for preventing the second sheet from being fed.
There may be a case in which sheets (particularly originals) stacked on a sheet stacker have a bent edge or are bound with a paper clip or staples. In such a case, even if the sheet separating mechanism described above is provided, it is difficult to securely separate sheets into a single sheet for feeding. For this reason, it is necessary to determine whether a single sheet is fed normally, or two sheets are overlapped and fed, described herein as ‘double feed’. Conventionally, an ultrasonic wave sensor has been known as a device in the art for detecting whether sheets are double fed.
In Japanese Utility Model Publication (Kokoku) No. 06-49567, an ultrasonic wave axis is formed of an ultrasonic wave sending device arranged above a transfer surface of a subject to be detected and an ultrasonic wave receiving device arranged below the transfer surface. The ultrasonic wave axis is arranged with an angle relative to the transfer surface, so that the ultrasonic wave receiving device detects a stable amount of an ultrasonic wave.
In Japanese Patent Publication (Kokai) No. 06-72591, an ultrasonic wave detector is arranged in a marking device disposed in a handover portion where a sheet is fed from a paper tray to a finishing apparatus. The ultrasonic wave detector is composed of a wave sending sensor and a wave receiving sensor arranged with the sheet in between, so that an ultrasonic wave is applied to the sheet with an angle to detect the double feed.
However, when a thin sheet is transported, the sheet tends to flap vertically. Accordingly, it is technically important to dispose an ultrasonic wave sensor at a proper position to receive an ultrasonic wave at a proper timing to accurately detect the double feed. Particularly, when an ultrasonic wave sensor is used for accurately detecting the double feed, a space between sheets becomes an important factor. If a gap is intentionally generated between sheets and a sensor detects the sheets after the sheets are extended in parallel to a traveling surface, it is possible to accurately detect the double feed of the sheets.
In the conventional ADF, an optical sensor for detecting a position of a sheet in a feeding path and an ultrasonic wave sensor for detecting the double feed of sheets are arranged separately at different locations. Based on output signals from the sensors, a timing of drawing a sheet is controlled, and a length of the sheet and the double feed are detected. In such a sheet handling apparatus having a function of detecting the double feed, it is necessary to separately dispose in a limited space an ultrasonic wave sensor for detecting the double feed and a sensor (corresponding to ‘register sensor’ disclosed in Japanese Patent Publication (Kokai) No. 2001-354339) for detecting a leading edge and a trailing edge of a sheet in a sheet path.
In view of the problems described above, the present invention has been made, and an object of the invention is to provide a sheet handling apparatus and an image reading apparatus capable of accurately detecting whether a sheet stacked on a sheet stacker is correctly fed one at a time without the double feed.
Another object of the invention is to provide a sheet handling apparatus and an image reading apparatus having one detection device capable of detecting both the double feed of sheets stacked on a sheet stacker to determine that the sheet is correctly fed one at a time, and a leading edge and/or a trailing edge of the sheet.
Further objects and advantages of the invention will be apparent from the following description of the invention.